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New analogue interface for LVDTs

 October 2001


New analogue interface for LVDTs

A new type of programmable analogue interface for LVDTs could be handy for machine builders. JON SEVERN reports

Linear variable displacement transducers (LVDTs) provide a simple and reliable means of converting mechanical displacement into an electrical signal. They do not suffer from electrical hysteresis and can be adapted to harsh environments. And because there are no discontinuities in the output signal, LVDTs have practically infinite resolution.

Unfortunately LVDTs require supporting analogue circuitry - an AC stimulus and output signal conditioning - that differs for each LVDT model. Ready-to-go solutions are available, but they add significant cost impact to the finished system. Now, however, all the LVDT support circuitry can be accomplished in a single inexpensive programmable analogue device, the flexible field programmable analogue array (FPAA).

An FPAA contains a matrix of 'configurable analogue blocks' consisting of switched-capacitor op-amp cells, surrounded by a programmable interconnect and I/O structure (see screenshot below). Common analogue signal conditioning functions can be implemented using just one cell, with more complex functions being implemented using two or more. In addition an FPAA contains internal voltage references and programmable clocks. Circuits are designed using a free CAD tool - AnadigmDesigner - without needing to know underlying circuit techniques and without breadboarding. Instead, users simply select analogue building blocks that are specified purely in terms of their functions, and enter the particular characteristic they need. AnadigmDesigner offers over 50 parameterisable analogue functions in its library including gain stages, amplifiers, sample and hold, filters, oscillators, rectifiers, comparators, DC references, limiters, peak detectors, Schmitt triggers, integrators, differentiators, and waveform generators. Experienced analogue designers can also use the chip's resources to create their own custom designs. The circuit's configuration is held in on-chip SRAM, which is initialised on power-up from EPROM, or through the chip's microprocessor peripheral interface.

An LVDT is a cylindrical transformer with one primary and two secondary windings (think of it as three solenoid windings stacked end to end). All the windings are wrapped around a hollow open-air core containing a non-contacting ferrous core. The displacement of an object can be measured if it is mechanically linked to the movable core.

If an AC signal is applied to the primary side and the ferrous core moved towards the 'B' side secondary windings, the coupling from the primary to the B side secondary will increase and the coupling to the A side secondary will decrease. It is possible to read the changes caused by this displacement by monitoring only Vb or Va, but it is better to perform a ratiometric measurement: Vout = (Va - Vb)/(Va + Vb). This removes the effects of stray noise, and fluctuations in the driving amplitude (which of course results in a larger signal being presented on both of the secondary windings, with no net gain).

Benefits of FPAAs

To build a typical circuit using conventional analogue components would need hours of design, construction, debug, trimming and tuning. Production would require a significant number of components to implement, consuming valuable board space and increasing manufacturing costs. And the design would be 'fixed forever'.

Consider the situation where an increase in sensitivity is required. With a discrete solution, you would be forced to re-design, restock inventory and possibly scrap work in progress. With the FPAA, only the receiver's gain settings need be adjusted to get more sensitivity.

  • Anadigm

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